Optimize power control in 3G handsets

We have moved into the "Fourth Wave of Computing." One where computation is less about raw horsepower and more about connectivity and power efficiency. In other words, we are moving from the era of PCs and into the era of smartphones and tablets. This transition will take us to an ever more intuitive and interactive experience with our electronic devices. From mobile apps to augmented reality, the next generation of computation devices are wirelessly-connected and always on.
Our everyday lives are becoming more dependent on these next generation devices and we expect them to be connected at all times. Advanced wireless standards such as WCDMA, WiFi, and LTE ensure that our products are interoperable with each other and around the world. We rely on these standards to ensure the robust operation of the mobile products we own.

Balancing TRP, SAR, and ID
In order to provide products that are reliable, fun to use and safe, mobile device designers strive to create products that can achieve high total radiated power (TRP). Consumers want devices with elegant industrial design (ID) and hidden antennas, all while simultaneously not violating radiated safety limits, measured as SAR (specific absorption rate). Balancing these three conflicting goals is no simple task for mobile device designers.

Wireless operators would prefer to see mobile devices with the highest TRP, ensuring the highest data rates and fewest dropped calls. Consumers make the buying decision based on other factors such as software interface and great ID. At the same time, government safety regulators must ensure that wireless devices in the market err on the side of safely, limiting wireless SAR emissions that are considered harmful, but possibly also limiting TRP.

A factor affecting these three design goals is the power control system of a mobile device. In 3G WCDMA and CDMA communications systems, very tight control of mobile device power is also required in order to ensure that received power level at the base station is the same for all devices in the cell. To ensure this tight control, the base station is constantly (at a rate of ~ every 500 us) sending commands to the mobile device to either increase or decrease output power.
Of course, as the mobile device moves close to the edge of the cell, the base station will continuously demand increased output power. At some point the mobile device itself must disregard the base station's request for higher power. Output power must be limited by the mobile device itself in order to ensure SAR safety.

Three methods to limit power
Mobile devices today use three methods to limit power to safe levels. The first method is "open-loop" power control. In this case, the mobile device is set to maximum power during production, and the internal transceiver power level (point A in Figure 1) is recorded. Once deployed in the field, the mobile device software ensures that the transceiver power (PXCVR) never exceeds the calibrated level.

This approach gives a good first-order approximation to the TRP of the device. However, the antenna faces a variety of loads in the real world not seen on the production line and some of the power can be reflected at the antenna, but not accounted for by the transceiver. Because the power is being measured at point A and not point G, open-loop power measurement is only a first-order approximation to the power that is actually delivered to the load (the air via the antenna).
The second method is "closed-loop" power control using a directional coupler. In this case, the power is measured at the output of the PA (point F in Figure 1). A directional coupler is conceptually a parallel wire that couples some of the RF energy from the output of the PA. Further, by using a quarter-wave length of wire, the two endpoints (or ports) of the coupler measure forward and reverse (or reflected) power.

By tapping both the forward ("coupled" ) and reverse ("isolated") ports of the coupler (points D and E in Figure 1), the mobile device can determine exactly how much power is delivered to the load (PDELIVERED = PFORWARD – PREVERSE). However, for reasons of both cost and practicality of implementation in high volume, just a single power detector is connected to the coupled port and only PFORWARD is measured in mobile devices that use closed-loop power control today.

The third method is closed-loop power control using a power detector integrated into the PA that directly measures PDELIVERED. An example is Black Sand Technologies' BST35 series of power amplifiers. Utilizing TrueDeliveredTM power detect technology, the BST35 series accurately measures PDELIVERED without a coupler via a patented on-chip power detector implemented in CMOS technology.

3G mobile device output power (point G on Figure 1) is typically calibrated to 24 dBm on the production line. Assuming 2.5 dB of insertion loss for the duplexer and antenna switch, this is equivalent to 26.5 dBm of output power from the PA (point F on Figure 1) and, assuming 26.5 dB of PA gain, this is equivalent to 0 dBm of output power from the transceiver (point A on Figure 1).

Walking through the calibration (done at 50 ohms) procedure for the three approaches in this example:Open-loop: The calibration would detect and record 0 dBm transceiver power as the power limit. (Point A in Figure 1)

Closed-loop with coupler: The calibration would detect the voltage from the power detector (point B in Figure 2) associated with 23.5 dBm output power (let's arbitrarily assume that ends up at 1.0 V). The power detector is connected to the coupled port of the coupler therefore measures forward power. At 50 ohms, forward power and delivered power are the same.

Closed-loop with integrated detection of delivered power: The calibration would detect the voltage from the integrated power detector (TrueDelivered™ power detector in the case of the BST35 series – point C in Figure 2) (let's also arbitrarily assume that ends up at 1.0 V).

Performance comparisons
Figure 2 compares the performance of the three approaches with mismatch of 17:1 VSWR at the antenna (with 2.5 dB of insertion loss, this produces a mismatch of 3:1 VSWR at the PA output – not uncommon in the real world). In each case, it is assumed that the basestation is constantly requesting higher power and that the power control system maintains the mobile device at calibrated maximum output power.

This means that:

for the open-loop case, transceiver power is maintained at 0 dBm;

for the closed-loop with coupler case, forward power detected is maintained at 1 V;

for the closed-loop with integrated delivered power detection case, delivered power detected is maintained at 1V.

Figure 2 shows the benefit of measuring PDELIVERED rather than PFORWARD or PXCVR. Measurement results using a PA in open loop (red), a PA with a high quality coupler (directivity = 20 dB) in closed loop (brown), a PA with a low-quality coupler (directivity = 5 dB) in closed loop (green), and Black Sand's BST3501 TrueDeliveredTM detector in closed loop (blue).

It can quickly be concluded that for closed loop to have a benefit over open-loop a certain level of coupler directivity performance is required and that both TRP and SAR performance are significantly degraded. Also, the mismatch error caused by using a coupler in closed-loop but only measuring PFORWARD clearly results in lower TRP even with a high quality coupler. The best way to maintain high TRP while still ensuring SAR safety is to implement a closed-loop power control that measures PDELIVERED.

As 3G mobile devices continue to become more essential to our daily lives, it is ever more important that they operate to the highest level of performance possible. By implementing closed-loop power control that relies on actual PDELIVERED, mobile device designers will be able to better balance TRP, ID, and SAR in the future.